A rare earth magnet such as samarium cobalt or cerium cobalt has been used as magnets for microwave tubes, for example as shown in U.S. Pat. No. 3,781,592. However, such devices have generally been positioned sufficiently far from sources of heat in the microwave tubes so that a relatively low temperature such as 125.degree. C. was not exceeded. As a result, additional weight of material for the pole piece and an additional amount of permanent magnet material was generally required. When rare earth permanent magnet material was used over an extended period of time in air even at temperatures somewhat below 125.degree. C., the magnet properties of the rare earth magnet were altered generally reducing the energy product and changing the operating characteristics of devices such as microwave tubes.